Compatible four channel fm system

ABSTRACT

A four channel FM system is described. In one embodiment the usual 19 kHz pilot signal is employed to switch between front and rear information, while in another, a 76 kHz switching signal is employed for this purpose.

States Patet 1 1 [111 3,822,365

Dorren 1 *July 2, 1974 COMPATIBLE FOUR CHANNEL FM [51] Int. Cl.111104111 5/00 SYSTEM [58] Field of Search 179/15 ET, 1 GQ [75]Inventor: Louis Dorren, M1llbrae, Ca11f. [56] References Cited [73]Assignee: Matsusliita Electric Corporation of UN STATES PATENTS New York3,679,832 7/1972 Halpern 179/15 BT Notice: The portion of the term ofthis 3,708,623 1/1973 Dorren 179/15 BT patent subsequent to Jan. 2,1990, has been disclaimed. Primary Examiner-Kathleen H. Claffy Filed Dec29 1972 Assistant ExaminerThomas DAmico [21] Appl. No.: 319,939 [57]ABSTRACT A four channel FM system is described. In one em- [63] CRelfted g ;g f 1970 P t bodiment the usual 19 kHz pilot signalisemployed to g fi t g switch between front and rear mformanon, wh11e 1n3 g g s g gi gg 0 another, a 76 kHz switcl ing signal is employed forthis purpose. [52] 11.8. CI. 179/15 HT 14 Claims, 15 Drawing FiguresLEFT RIGHT REAR REAR LEFT RIGHT FRONT FRONT Z0 *22 QUADRAPLEX QUADRAPLEX'19 17 ENCODEK ENCODEK 13x 15 |*1I80 8 o 26 U W LEFT RIGHT MULTIPLEX /8EXCITEK 1 Z8 19 kHz TRANSMITTER mmmm 2:914 1822.365

SHEU 1 [If 5 LEFT RIGHT REAR REAR EFT klGHT FRONT FRONT LEFT LEFT momREAR W/WQUADKAPLEX QUADRAPLEXA/W f! m ENCODEK ENCODEK g 1 A5 0' 24 18 UW180 +-+1so 25 J M M WW y I3 i A bus air/m /8 in? F I a 1 'Z8 [9 kHzTRANSMITTEKJ LEFT LEFT W 53? 2% 5w 32%@@% f k F I E- q: 44 RIGHT KEAQKSMENFWJHL 2 m4 SHEET 2 OF 5 {5CA j 0 HK5T ECOND "180 IW tfi g LEF LEFTWIT f UPPER SIDEBAND LEFT 38 k Hz WT -LOWEK SIDEBAND RIGHT FRONT KEAKmom I9 kHz mm i g w-F/K5T /30-M-5EC0ND 160 KHZ 2: LF RF LR RR 0 kHzPmmwmm 21w 3,822,366

sum 3 or s L FT KI HT KQAK K AR Y L T RIGHT FR FROM T PATENTEUJUL 2 I974LEFT RIGHT REAR REAR LEFT FRONT RIGHT FRONT 76 kHz OSCILLATOR AUDIODIV/DER 38 kHz GATES LEFT- RIGHT PILOT FRONT REAR Y FIKST 51/5- CHANNELY saw/v0 SUB CHANNEL I am . I COMPATIBLE FOUR CHANNEL FM SYSTEM CROSSREFRERENCE TO RELATED APPLICATIONS This application is a continuingapplication of US. Pat. No. 3,708,623 issued Jan. 2, 1973 which wasapplication Ser. No. 32,989, filed Apr. 29, 1970, which, in turn, was acontinuation-in-part of my application Ser. No. 13,902, filed Feb. 25,1970 now abandoned.

SUMMARY OF THE INVENTION A four channel audio system is provided whichis fully compatible with standard FM stereo and mono equipment.According to the present invention, a switching or sampling system isemployed so that four audio channels are transmitted by the FM station.These four channels are designated left front, right front, left rearand right rear and are sometimes hereinafter abbreveiated LF, RF, LR andRR, respectively.

In accordance with one embodiment of the invention, these four channelsare superimposed on the 38 kHz subcarrier and the usual 19 kHz pilotsignal is used as a switching signal. During the first half cycle of the19 kHz signal, the left front and right front information is transmittedwhile during the second half cycle, the left rear and right rearinformation is transmitted. The 19 kHz pilot is then used as a switchingsignal between the front and rear information as is hereinafterdescribed in detail while the 38 kHz signal switches between left andright in the usual manner. This system has the advantage of notrequiring an increase in bandwidth, not requiring any additional pilotor subcarrier frequencies and permits the radio station to continue touse its normal 67 kHz subcarrier for SCA purposes. However, thisembodiment does not ordinarily permit the use of the full frequencyspectrum for front and back information so that it is sometimespreferable to provide another subcarrier to carry the front and rearinformation. When this is done, the 19 kHz pilot signal is quadrupledand this quadrupled signal is used for switching between front and backinformation.

Both embodiments of the present invention are completely compatible withpresent mono and stereo equipment. The main channel carries all fouraudio channels so that on a mono receiver the four signals are combined.On a stereo receiver, the left and the right information is extracted inthe usual manner and it is only with the receiver equipped for fourchannel reception that the signal produced by the system of the presentinvention is broken into its four components. The system utilizing the76 kHz switching signal has the additional advantage that there is acomplete reproduction of the full audio bandwidth by each of the fourchannels.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of atransmitter embodying the present invention.

FIG. 2 is a block diagram of a receiver embodying the present invention.

FIG. 3 is an analogy diagram of one of the encoders shown in FIG. 1.

FIG. 4 is a similar diagram of one of the decoders utilized in FIG. 2.

FIG. 5 is a spectrum diagram of the signal employed during thetransmission of front channel-information.

FIG. 6 is a similar diagram showing the signal employed during thetransmission of rear channel information.

FIG. 7 is a diagram of the composite wave form employed.

FIG. 8 is a block diagram showing how combiner networks can be employedwith the encoders to eliminate beat notes.

FIG. 9 is a block diagram showing a limiter circuit which is desirableto employ with certain types of FM receivers.

The above FIGS. 1 through 9 relate to an embodiment wherein the 19 kHzpilot signal is employed to switch between front and rear information.The following figures relate to that embodiment of the invention whereina 76 kHz subcarrier and switching signal is employed.

FIG. 10 is a wave form of that embodiment of the invention wherein a 76kHz subcarrier is employed.

FIG. 11 is a band distribution diagram of the system using the 76 kHzsubcarrier.

FIG. 12 is a block diagram of the encoder employed with the 76 kHzsubcarrier system.

FIG. 13 is a block diagram of the receiver employed with the system.

FIG. 14 is a switch analogy for the transmitter.

FIG. 15 is a switch analogy for the receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is made to FIGS. 1through 9 which illustrate that embodiment of the invention employing a19 kHz switching signal.

At the present time, the authorized stereophonic system used in theUnited States includes a multiplex signal wherein left and right channelinformation is carried on a single carrier wave. The composite signalincludes a main channel signal on the carrier frequency which containsboth the right and left hand signals. A suppressed carrier double sideband signal is provided on a subcarrier at 38 kHz which carries the leftminus right signal. A 19 kHz pilot signal is provided for phase lock ofthe 38 kHz side bands. In addition, there may be a subsidiarycommunication authorization (S.C.A.) signal at 67 kHz and this systemdoes not interfere with the SCA. signal. In general, this embodiment ofthe invention is carried out by employing the 19 kHz pilot signal as aswitching signal to switch between front and back information to providefor front and back as well as left and right signals.

The method of transmitting the signal is shown in FIGS. 1 and 3. Astandard multiplex exciter 8 is employed, and this generates a 19 kHzpilot signal which serves as the switching signal for the front and rearinformation. This signal is passed through lead 10 to diodes 12 and 14and alternate half cycles are passed by leads l3 and 15 to the encodersl7 and 19. One of these encoders is shown in analog form in FIG. 3 andconsists of switches 16 and 18. The encoder has a line 20 leading toswitch 16 for front information and a line 22 leading to switch 18 forrear information and a common output line 24. The switches are actuatedby pulses from lines 13 and 15 respectively. Thus, on a positive halfcycle of the 19 kHz signal, switch 16 is closed so that the frontinformation is fed through line 24 while on the following negative halfcycle, switch 18 is closed so that the rear information is fed throughline 24. The other encoder is not described since it operates in exactlythe same way with the right front and rear information. The leftinformation is fed through line 24 to the multiplex exciter 8 while thecorresponding right channel information is fed through line 26 to theexciter. The signal generated by the exciter is then fed to thetransmitter 28.

The signal thus generated can best be understood by reference to FIGS.and 6. FIG. 5 represents the signal as it is being transmitted duringthe first or positive half cycle of the 19 kHz signal. It will be seenthat a O frequency (zero in this sense represents the nominal carrierfrequency of the FM station) there is present up to kHz, a signalrepresenting the left front plus right front information. Centered on 38kHz is the left front minus right front information. In FIG. 6 thesignal is shown during the second or negative half cycle of the 19 kHzsignal. During this half cycle, the main channel is carrying the leftplus right information but in this case it is the rear information whilesimilarly centered on 38 kHz the left minus right information is sentbut here again it is the rear information. It will be seen that in bothinstances, there is no interference with the normal S.C.A. signal.

The method of receiving a signal is shown in FIGS. 2 and 4. The signalis received on an ordinary FM tuner 30 and fed to a standard multiplexdemodulator 32. The 19 kHz signal is taken from line 34 and passed todiodes 36 and 38 and the rectifier I9 kHz pulses are passed throughlines 40 and 42 to the decoders 44 and 46. The left and right channelinformation is taken from the demodulator 32 and passed through lines 48and 50 to the respective decoders. Referring now specifically to FIG. 4,the upper switch is actuated by the positive half cycles from line 40while the lower switch is operated by the negative half cycles from line42 so that the decoder switches between the front and the right front onpositive and negative pulses of the 19 kHz signal. The signals are thentaken from the decoder and amplified in the usual way. At the same time,the ight channel information is handled in the same manner by thedecoder 46.

In order to prevent high frequency components of the incoming programinformation from interfering with operation of the system, low passfilters which pass only frequencies below 15 kHz are employed in theinput leads to the encoders, i.e., leads and 22 of encoder 17 and thecorresponding leads of the right hand encoder 19. Similar low passfilters are also employed in the output leads of decoders 44 and 46.

The operation of the overall system can best be seen in FIG. 7 whereinit is assumed that demodulation takes place on a time division basisalthough it will be obvious that the invention ie equally applicable todemodulators which operate on a matrixing system or a combination oftime division and matrixing. FIG. 7 represents one complete cycle of the19 kHz signal and two complete cycles of the 38 kHz subcarrier. Duringthe first half of the I9 kHz cycle, front information exclusively isbeing sent both on the main channel and on the 38 kHz subcarrier. In thecase of the main channel, this is a combination of the left front andright front information while on the 38 kHz subcarrier, left frontinformation is being sent on the upper side band and right frontinformation on the lower side band. During the next half cycle of the 19kHz signal, the situation is reversed with the main channel carryingleft rear plus right rear information while on the 38 kHz signal, leftrear information is carried on the upper side band while right rearinformation is carried on the lower side band.

Although the embodiment heretofore described is a fully workable system,some modifications can be made for optimum results.

When the full bandwidth of 15 kHz is transmitted with this signal, a 15kHz audio tone will produce sidebands of 4 kHz and 34 kHz when imposedon the 19 kHz pilot signal. The 34 kHz signal will beat with sidebandcomponents of the 38 kHz subchannel causing frequencies in the sub andmain channels, and the 4 kHz signal will beat with the main channelaudio components, creating beat frequencies lying within the audiblerange. Thus, to employ the full 15 kHz bandwidth, it is desirable toprovide a combining network on the inputs to the encoder as is shown inFIG. 8. Here the left channel encoder 17 having the inputs 20 and 22,previously described, has a combining network 52 connected between theinput lines. Obviously the right channel is treated the same way. Thiscombining network combines all frequencies above 4 kHz so that for thehigher frequencies, the information is carried on both the front and therear channels. At frequencies under 4 kHz there is a separation betweenfront and back. This gives very good presence since it has been foundthat a great deal of the separation presence occurs below 4 kHz.

With some relatively inexpensive FM receivers, some distortion may beencountered for the reason that such receivers employ a 19 kHz tunedamplifier together with a doubler. The 19 kHz signal in the case of suchreceiver is not a clean 19 kHz but has certain modulation componentsimposed thereon. In other words, the 19 kHz switching signal containsamplitude modulated components so that switching does not always takeplace at the exact points desired. In order to remedy this, a limiter asis shown in FIG. 9 may be employed with such receivers. Here the tuner54 feeds a signal through line 56 to the audio gates 58 and at the sametime the signal is fed to the 19 kHz tuned amplifier 60. The 19 kHzsignal is now passed to the limiter and doubler 62. The limited 19 kHzsignal is passed through line 64 to the audio gates to perform theswitching function and, since it is now free of all amplituded modulatedcomponents, provides a clean switching action. The doubled signal is fedto the 38 kHz amplifier 66 which serves as the switching signal for theright and left information in the usual manner. Most FM receivers do notrequire this added circuitry, particularly the better grade of receiverswhich use a phase locked oscillator rather than the simple tunedamplifier and doubler.

The same basic system is used in the scheme shown in FIGS. 10 through 15except here instead of using the 19 kHz signal to switch between frontand rear information, the 19 kHz signal is first doubled in the usualmanner to act as a switching signal for the left and right informationand again doubled to produce a 76 kHz signal which serves to switch thefront and rear information. In order to preserve compatibility, theorder in which the audio signals are transmitted is changed to LR, LF,RR and RF. It is also necessary to make a change in the bandwidth tohandle the system and in the specific system described, this must be atleast 91 kHz for the four channel transmission and it may be increasedto kHz to handle S.C.A. subcarrier.

In FIG. Ill, one half of the composite signal is shown. Thus, there is amain channel in the usual manner extending from 50 Hz to kHz and thiscontains the left plus right information, both front and rear. There isa pilot signal at 19 kHz and a first subchannel centered on 38 kHz. Thisfirst subchannel contains the left minus right information, includingboth front and rear. A second subchannel is centered on 76 kHz and thiscontains the front minus rear information. Summing up the above, it canbe seen that the novel composite signal of the present inventionincludes the following:

A main channel extending up to 15 kHz and including the sum of thesignals, for example, left and right both front and rear.

A 19 kHz pilot signal. A first sub-channel centered on 38 kHz containingleft minus right information.

A sub-channel centered on 76 kHz containing front minus rearinformation.

If this signal is studied, purely mathematical Fourier analysis showsthat the following signal equations actually exist. The first of thoseequations is assigned to the main channel and is the sum of the signals,i.e., (LF LR RF RR); this main channel extending up to 15 kHz. Thesecond equation is (LF LR RF RR) and is located in the first sub-channelwhich is centered at 38 kHz. Also located in the first sub-channel isanother equation (LF LR RF RR). To differentiate these two equations,they are modulated in quadrature; that is, the first being the sine of38 kHz and the second being the cosine of 38 kHz. The final equation iscentered at 76 kHz is (LF- LR RF RR).

With the production of these four equations in the base band signal, wehave now satisfied the algebraic conditions of transmission. The purelymathematical Fourier analysis is as follows:

ANALYTICAL DESCRIPTION OF THE DORREN QUADRAPLEX COMPOSITE SIGNALANALYSIS: THE MODULATING FUNCTION OF FOUR CHANNELS The modulatingfunctions of four channels are assumed to vary in the ways shown below.

The origin of the time scale is chosen as the beginning of one of thesampling pulses:

sl (r) s2 (r) =sl (r z/4) s3 (r) =s2 (t 1/4) s4 (1) si (t) is a periodicfunction with a fundamental frequency 2f and has a fourier seriesrepresentation:

E m 4 n1r 81(6) {1+ a 8111 cosin 41rnft+ 2 (1 cosin ZL15) sin 41rnfL}'rt1r 2 Retaining only the 38 kilohertz and 76 kilohertz components andapplying the relations of one to generate the other three functions, weget:

s1 (t) =E/4 l 4/1r cosine 41rft 4/11- sine 41rft+ 81rft.}

s2 (t) E/4 {l 4/11 cosine 41rft 4111 sine 41-rft 4/1r sine 81119. I s3(1) E/4{ 1 4/1r cosine 41rft 4/11 sine 41rft 4/17 sine 81'rft.}s4 (t)E/4{ l 4/11 cosine 41rft 4/17 sine 41rft 4/12 sine 87rft.} Multiplexingsi (t) by ai (t) and summing to give the composite, we get:

To this signal a pilot should be added of the form A sin 21rfr. We seethat the quadraplex composite signal consists of:

1. a main channel component (al a2 a3 a4).

2. two 38 kilohertz components in quadrature, one modulated by (al a2 a3a4) and the other modulated by (01 a2 a3 a4).

3. one 76 kilohertz component modulated by (al If we make the followingchannel identification,

a1 left front signal a2 left rear signal a3 right front signal a4 rightrear signal and assume the two channel stereo case in which,

al a2 left a3 a4 right the composite signal reduces to c (t) E/4 (21 2r)A sin 21Tft (21 2r) sin 41rft. This is the standard two channel stereoformat with the pilot at 19 kilohertz and having the correct phaserelationship to the 38 kilohertz subcarrier.

This composite signal is generated by the transmitter circuit as shownin FIG. 12 and the switch analogy as shown in FIG. 14 wherein the switchanalogy switches at the rates of 76 kHz and 38 kHz.

If an S.C.A. signal is desired, this can be centered on 105 kHz,although the provision for such a signal forms no part of the presentinvention.

In FIG. 10, the sampling system is shown. There is shown one full cycleof the 38 kHz subcarrier and naturally two cycles of the newly generated76 kHz subcarrier. The 38 kHz signal is utilized by a stereophonicreceiver in the usual manner so that the sampling points would be atpoints 68 and 70 for the left and right handed information. Similarly,the signal centered on the 76 kHz subcarrier contains the front minusrear information so that on a four channel audio system the samplingpoints would be at 72, 74, 76 and 78 to extract the desired information.

In FIG. 13 there is shown a block diagram of how the decoder works. Thecomposite signal comes from the tuner through line 80 and a portion ofthe composite signal goes to the 19 kHz amplifier 82. The compositesignal is also fed to the audio gates 84. The 19 kHz signal is doubledto 38 kHz in the doubler 86 and this signal is passed through line 88 tothe audio gate 84 where it is used to switch between the right and leftinformation. A portion of the signal is also sent to the second doubler90 which puts out a signal at 76 kHz through line 92 which is also sentto the audio gates and utilized to switch between front and backinformation.

The transmitter circuit is shown in FIG. 12 and essentially consists ofthe opposite circuit from that described for the decoder. Thus, foursources of audio are supplied through the four lines 94A, B, C and D tothe audio gates 96. A 76 kHz oscillator 98 is provided which sends asignal to the audio gates for switching between the front and the backinformation. The signal is divided by two in divider 100 and a portionof this 38 kHz signal is sent to the audio gates for switching betweenright and left information while a portion of this signal is sent to thedivider 102 for the generation of the 19 kHz pilot signal. The pilotsignal is combined with the composite signal from the audio gates toproduce a composite signal on 104 which can be used to modulate astandard FM transmitter. Naturally this signal will look like the signalof FIG. 11 except that no description has been included of thegeneration of the SCA band.

FIG. 14 shows a mechanical switch analogy of the switching circuit andthis as well as FIG. should be utilized in conjunction with FIG. 10.Here switch 104 operates at a frequency of 38 kHz for sampling the rightand left information while switch 106 operates at twice this frequencyfor sampling the left rear and left front information while switch 108operates at the same frequency for the same purpose in the rightchannel. Thus one can visualize switch 104 in the upper portion whileswitches 106 and 108 are also in the upper position. Switch 108 is ineffect inoperative since the right channel is open but switch 106switches the left rear information into the outgoing signal. Now switch106 (as well as 108) moves to the lower position so that the left frontinformation is sampled. After one complete cycle of the 76 kHz, switch104 moves to the lower position while switch 108 repeats the operationfor sampling the right front and rear information. FIG. 15 gives asimilar analogy for the receiver where switch 110 operating at afrequency of 38 kHz switches between right and left information whileswitches 112 and 114 similarly switch between front and rearinfonnation. Naturally, these are only mechanical analogies and in anormal receiver or transmitter such switching is by solid state devices.

It will be apparent from this description that the signal is completelycompatible with either a mono, stereo, or four channel receiver. Thus,on a mono receiver, one would hear the main channel which during twocycles of the 76 kHz subcarrier will contain the information from allfour channels. On a stereo receiver, left front and left rearinformation will be extracted during the first 180 period of the 38 kHzsubcarrier while the right front and right rear information will bereceived during the second 180 period. On the four channel receiver, thefour signals would be individually received as previously described.

It will be seen from the description which has been given that thecomplete signal of the first embodiment has been contained within theassigned bandwidth and that there has been no interference with anS.C.A. signal, if this is being sent. in the second embodiment the foursignals are all modulated to the full 15 kHz bandwidth so that there isno deterioration of separation over this bandwidth. The four signals arealso given the same percentage of modulation so there is nodeterioration of the signal to noise ratio.

1 claim:

1. In a four channel broadcasting system, a method for providing acompatible four channel composite signal for use in conjunction with anFM radio transmitter having said composite signal modulated on thecarrier thereof and consisting of a main channel, a synchronizing pilotsignal of a first predetermined frequency, a first sub-channel centeredat the second harmonic of said pilot signal and a second sub-channelcentered at the fourth harmonic of said pilot signal comprising thesteps of: providing first, second, third and fourth audio signals;providing a pilot signal of a first predetermined frequency; providing afirst sub-carrier frequency signal at twice the frequency of said pilotsignal; providing a second sub-carrier frequency signal at quadruple thefrequency of said pilot signal; applying said first and second audiosignals to a first audio gate and switching said first gate at thefrequency of said second subcarrier frequency signal; applying saidthird and fourth audio signals to a second audio gate and switching saidsecond audio gate at the frequency of said second subcarrier frequencysignal; applying the outputs from said first and second audio gates to athird audio gate and switching said third audio gate at the frequency ofsaid first sub-carrier frequency signal; and combining the output ofsaid third audio gate with said pilot to obtain said composite signal.

2. In a four channel receiving system, a method for decoding first,second, third and fourth audio signals from a compatible four channelcomposite signal adapted to be received from an FM radio transmitterhaving said composite signal modulated on the carrier thereof, saidcomposite signal consisting of a main channel, a synchronizing pilotsignal of a first predetermined frequency, a first sub-channel centeredat the second harmonic of said pilot signal and a second subchannelcentered at the fourth harmonic of said pilot signal, comprising thesteps of: extracting said pilot signal; providing a first switchingsignal at twice the frequency of said pilot signal; providing a secondswitching signal at quadruple the frequency of said pilot signal;applying said composite signal to a first audio gate and switching saidfirst audio gate at a frequency equal to the frequency of said firstswitching signal to provide a first gate output signal and a second gateoutput sig nal; applying said first gate output signal to a second audiogate and switching said second audio gate at a frequency equal to thefrequency of said second switching signal to provide said first andsecond audio signals; and applying said second gate output signal to athird audio gate and switching said third audio gate at a frequencyequal to the frequency of said second switching signal to provide saidthird and fourth audio signals.

3. In a four channel broadcasting system, a method for providing acompatible four channel composite signal for use in conjunction with anFM radio transmitter for transmitting said composite signal, whichcomposite signal includes a main channel with first, plus second, plusthird and plus fourth audio signals combined thereon, a pilot signal ofa predetermined frequency removed from said main channel, a firstsuppressed subchannel centered on a frequency equal to the secondharmonic of said pilot signal, said first sub-channel having first, plussecond, minus third, minus fourth audio signals and in quadrautretherewith first, minus second, minus third and plus fourth audio signalscombined thereon and a second suppressed sub-channel centered on afrequency twice that of said first sub-channel having first, minussecond, plus third, minus fourth audio signals combined thereon,comprising the steps of: providing a pilot signal of a firstpredetermined frequency; providing a first switching signal at twice thefrequency of said pilot signal; providing a second switching signal 9 atquadruple the frequency of said pilot signal; utilizing said secondswitching signal to switch between said first and second audio signalsto obtain a first signal; utilizing said second switching signal toswitch between said third and fourth audio signals to obtain a secondsignal; utilizing said first switching signal to switch between saidfirst and second signals to obtain a third signal; combining said thirdsignal with said pilot signal to obtain said composite signal; andmodulating said F M transmitter with said composite signal.

4!. The method according to claim 3 further comprising the steps of:providing a main channel extending up to a frequency less than thefrequency of said pilot signal and having first, plus second, plusthird, plus fourth audio signals combined thereon; providing asuppressed sub-channel centered at twice the frequency of said pilotsignal and having first, plus second, minus third, minus fourth audiosignals and in quadrature therewith first, minus second, minus third andplus fourth audio signals combined thereon; and providing a secondsuppressed sub-channel at quadruple the frequency of said pilot signaland having first, minus second, plus third, minus fourth audio signalscombined thereon.

5. In a four channel receiving system, a method for deriving first,second, third and fourth audio signals from a compatible four channelcomposite signal received from an F M radio transmitter, which compositesignal includes a main channel with first, plus second, plus third andplus fourth audio signals combined thereon, a pilot signal having apredetermined frequency removed from said main channel, a firstsuppressed sub-channel which is centered at the second harmonic ofsaid-pilot signal, said first sub-channel having first, plus second,minus third, minus fourth audio signals and in quadrature therewithfirst, minus second, minus third and plus fourth audio signals combinedthereon and a second suppressed sub-channel centered at the fourthharmonic of said pilot signal and having first, minus second, plusthird, minus fourth audio signals combined thereon comprising the stepsof: deriving a signal equal in frequency to the frequency of said pilotsignal; deriving a first switching signal equal in frequency to twicethe frequency of said pilot signal; deriving a second switching signalequal in frequency to quadruple the frequency of said pilot signal;utilizing said first switching signal to switch said composite signal attwicethe frequency of said pilot signal to obtain a first signal and asecond signal; utilizing said second switching signal to switch saidfirst signal at quadruple the frequency of said pilot signal to obtainsaid first and second audio signals; and utilizing said second switchingsignal to switch said second signal at quadruple the frequency of saidpilot signal to obtain said third and fourth audio signals.

6. In a four channel broadcasting system, a method for providing a fourchannel composite signal compatible with existing mono and stereostandards for use in conjunction with an F M radio transmitter having apredetermined carrier frequency, comprising the steps of: providing amain channel extending up to a first predetermined frequency and havingfirst, plus second, plus third and plus fourth audio signals combinedthereon; providing a pilot signal of a second predetermined frequencyremoved from said carrier frequency; providing a first sub-channelcentered at the second harmonic of the frequency of said pilot signaland containing two sub-carriers; modulating the first sub-carrier ofsaid first sub-channel with the plus first, plus second, minus third,minus fourth audio signals and modulating in quadrature therewith thesecond sub-carrier of said first sub-channel with plus first, minussecond, minus third and plus fourth audio signals; providing a secondsub-channel centered at the fourth harmonic of the frequency of saidpilot signal and containing a sub-carrier; and modulating thesub-carrier of said second subchannel with the plus first, minus second,plus third and minus fourth audio signals.

7. In a four channel broadcasting system, a system for providing acompatible four channel composite signal for use in conjunction with anFM radio transmitter having said composite signal modulated on thecarrier thereof and consisting of a main channel, synchronizing pilotsignal of a first predetermined frequency, a first sub-channel centeredat the second harmonic of said pilot signal and a second sub-channelcentered at the fourth harmonic of said pilot signal comprising: meansfor providing first, second, third and fourth audio signals; means forproviding a pilot signal of a first predetermined frequency; means forproviding a first subcarrier frequency signal at twice the frequency ofsaid pilot signal; means for providing a second sub-carrier frequencysignal at quadruple the frequency of said pilot signal; a first audiogate; means for applying said first and second audio signals to saidfirst audio gate; means for switching said first audio gate at thefrequency of said second sub-carrier frequency signal; a second audiogate; means for applying said third and fourth audio signals to saidsecond audio gate; means for switching said second audio gate at thefrequency of said second sub-carrier frequency signal; a third audiogate; means for applying the outputs from said first and second audiogates to said third audio gate; means for switching said third audiogate at the frequency of said first sub-carrier frequency signal; andmeans for combining the output of said third audio gate with said pilotsignal to obtain said composite signal.

8. In a four channel receiving system, a system for decoding first,second, third and fourth audio signals from a compatible four channelcomposite signal adapted to be received from an FM radio transistorhaving said composite signal modulated on the carrier thereof, saidcomposite signal consisting of a main channel, a synchronizing pilotsignal of a first predetermined frequency, a first sub-channel centeredat the second harmonic of said pilot signal and a second sub-channelcentered at the fourth harmonic of said pilot signal, comprising: meansfor extracting said pilot signal from said composite signal; means forproviding a first switching signal at twice the frequency of said pilotsignal; means for providing a second switching signal at quadruple thefrequency of said pilot signal; a first audio gate; means for applyingsaid composite signal to said first audio gate; means for switching saidfirst audio gate at a frequency equal to the frequency of said firstswitching signal to provide a first gate output signal and a second gateoutput signal; a second audio gate; means for applying said first gateoutput signal to said second audio gate; means for switching said secondaudio gate at a frequency equal to the frequency of said secondswitching signal to provide said first and second audio signals; a thirdaudio gate; means for applying said second gate output signal to saidthird audio gate; and means for switching said third audio gate at afrequency equal to the frequency of said second switching signal toprovide said third and fourth audio signals.

9. In a four channel broadcasting system, a system for providing acompatible four channel composite signal for use in conjunction with anFM radio transmitter for transmitting said composite signal, whichcomposite signal includes a main channel with first, plus second, plusthird and plus fourth audio signals combined thereon, a pilot signal ofa predetermined frequency removed from said main channel, a firstsuppressed subchannel centerd on a frequency equal to the secondharmonic of said pilot signal, said first sub-channel having first, plussecond, minus third, minus fourth audio signals and in quadraturetherewith first, minus second, minus third and plus fourth audio signalscombined thereon and a second suppressed sub-channel centered on afrequency twice that of said first sub-channel having first, minussecond, plus third, minus fourth audio signals combined thereon,comprising: means for providing a pilot signal of a first predeterminedfrequency; means for providing a first switching signal at twice thefrequency of said pilot signal; means for providing a second switchingsignal at quadruple the frequency of said pilot signal; means forswitching between said first and second audio signals at quadruple thefrequency of said pilot signal to obtain a first signal; means forswitching between said third and fourth audio signals at quadruple thefrequency of said pilot signal to obtain a second signal; means forswitching between said first and third signals at twice the frequency ofsaid pilot signal to obtain a third signal; and means for combining saidthird signal with said pilot signal to obtain said composite signal.

10. The system according to claim 9 wherein said composite signalincludes a main channel extending up to a frequency less than thefrequency of said pilot signal and having first, plus second, plusthird, plus fourth audio signal combined thereon; a suppressedsubchannel centered at twice the frequency of said pilot signal andhaving first, plus second, minus third, minus fourth audio signals andin quadrature therewith first, minus second, minus third and plus fourthaudio signals combined thereon; and a second suppressed subehannel atquadruple the frequency of said pilot signal and having first, minussecond, plus third, minus fourth audio signals combined thereon.

11. In a four channel receiving system, a system for deriving first,second, third and fourth audio signals from a compatible four channelcomposite signal received from an FM radio transmitter, which compositesignal includes a main channel with first, plus second, plus third andplus fourth audio signals combined thereon, a pilot signal having apredetermined frequency removed from said main channel, a firstsuppressed sub-channel which is centered at the second harmonic of saidpilot signal, said first sub-channel having first, plus second, minusthird, minus fourth audio signals and in quadrature therewith first,minus second, minus third and plus fourth audio signals combined thereonand a second suppressed sub-channel centered at the fourth harmonic ofsaid pilot signal and having first, minus second, plus third, minusfourth audio signals combined thereon comprising: means for deriving asignal equal in frequency to the frequency of said pilot signal; meansfor deriving a first switching signal equal in frequency to twice thefrequency of said pilot signal; means for deriving a second switchingsignal equal in frequency to quadruple the frequency of said pilotsignal; means for utilizing said first switching signal to switch saidcomposite signal at twice the frequency of said pilot signal to obtain afirst signal and a second signal; means for utilizing said secondswitching signal to switch said first signal at quadruple the frequencyof said pilot signal to obtain said first and second audio signals; andmeans for utilizing said second switching signal to switch said secondsignal at quadruple the frequency of said pilot signal to obtain saidthird and fourth audio signals.

12. In a four channel broadcasting system, a system for providing a fourchannel composite signal compatible with existing mono and stereostandards for use in conjunction with an FM radio transmitter having apredetermined carrier frequency, comprising: means for providing a mainchannel extending up to a first predetermined frequency and havingfirst, plus second, plus third and plus fourth audio signals combinedthereon; means for providing a pilot signal of a second predeterminedfrequency removed from said carrier frequency; means for providing afirst sub-channel centered at the second harmonic of the frequency ofsaid pilot signal and containing two sub-carriers; means for modulatingthe first sub-carrier of said first sub-channel with the plus first,plus second, minus third, minus fourth audio signals and for modulatingin quadrature therewith the second sub-carrier of said first sub-channelwith plus first, minus second, minus third and plus fourth audiosignals; means for providing a second sub-channel centered at the fourthharmonic of the frequency of said pilot signal and containing asubcarrier; and means for modulating the sub-carrier of said secondsub-channel with the plus first, minus second, plus third and minusfourth audio signals.

13. A compatible four channel audio system for providing a compatiblefour channel composite signal comprising means for providing left front,left rear, right front and right rear audio signals, means for providinga 19 kHz pilot signal, means for providing a 38 kHz sub-carrier signal,first circuit means coupled to said left front and left rear audiosignals for combining all frequencies of said left front and left rearaudio signals above 4 kHz and for providing separation between said leftfront and left rear audio signals at frequencies less than 4 kHz, firstencoder means coupled to said first circuit means and said left frontand left rear audio signals, said first encoder means including firstswitch means responsive to said pilot signal for providing a firstencoder output during the positive half cycle of said pilot signal andsecond switch means responsive to said pilot signal for providing asecond encoder output during the negative half cycle of said pilotsignal, second circuit means coupled to said right front and right rearaudio signals for combining all frequencies of said right front andright rear audio signals above 4 kHz and for providing separationbetween said right front and right rear audio signals at frequenciesless than 4 kHz, second encoder means coupled to said second circuitmeans and said right front and right rear audio signals, said secondencoder means including first switch means responsive to said pilotsignal for providing a first encoder output during the positive halfcycle of said pilot signal and said second switch means responsive tosaid pilot signal for providing a second encoder output during thenegative half cycle of said pilot signal, means for combining the outputsignals from said first and second encoder means to provide saidcomposite signal, and transmitter means for transmitting said compositesignal, said transmitter means responsive to said 38 kHz sub-carriersignal for transmitting for transmitting during the'positive cycles ofsaid subcam'er signal a main channel having left front plus right frontaudio signals modulated thereon, said 19 kHz pilot signal and asuppressed sub-channel having left front minus right front audio signalsmodulated thereon and for transmitting during the negative cycles ofsaid sub-carrier signal a main channel having left rear plus right rearaudio signals modulated thereon, said 19 kHz pilot signal and asuppressed sub-channel having left rear minus right rear signalsmodulated thereon 114. The system according to claim 13 furtherincluding means for receiving said composite signal, means forextracting said 19 kHz pilot signal from said commeans adapted toreceive said right front and right rear audio signals, said seconddecoder means including first switch means responsive to said pilotsignal for providing said right front audio signal during the positivehalf cycle of said pilot signal and second switch means responsive tosaid pilot signal for providing said right rear audio signal during thenegative half cycle of said pilot signal.

1. In a four channel broadcasting system, a method for providing acompatible four channel composite signal for use in conjunction with anFM radio transmitter having said composite signal modulated on thecarrier thereof and consisting of a main channel, a synchronizing pilotsignal of a first predetermined frequency, a first sub-channel centeredat the second harmonic of said pilot signal and a second sub-channelcentered at the fourth harmonic of said pilot signal comprising thesteps of: providing first, second, third and fourth audio signals;providing a pilot signal of a first predetermined frequency; providing afirst subcarrier frequency signal at twice the frequency of said pilotsignal; providing a second sub-carrier frequency signal at quadruple thefrequency of said pilot signal; applying said first and second audiosignals to a first audio gate and switching said first gate at thefrequency of said second sub-carrier frequency signal; applying saidthird and fourth audio signals to a second audio gate and switching saidsecond audio gate at the frequency of said second subcarrier frequencysignal; applying the outputs from said first and second audio gates to athird audio gate and switching said third audio gate at the frequency ofsaid first sub-carrier frequency signal; and combining the output ofsaid third audio gate with said pilot to obtain said composite signal.2. In a four channel receiving system, a method for decoding first,second, third and fourth audio signals from a compatible four channelcomposite signal adapted to be received from an FM radio transmitterhaving said composite signal modulated on the carrier thereof, saidcomposite signal consisting of a main channel, a synchronizing pilotsignal of a first predetermined frequency, a first sub-channel centeredat the second harmonic of said pilot signal and a second sub-channelcentered at the fourth harmonic of said pilot signal, comprising thesteps of: extracting said pilot signal; providing a first switchingsignal at twice the frequency of said pilot signal; providing a secondswitching signal at quadruple the frequency of said pilot signal;applying said composite signal to a first audio gate and switching saidfirst audio gate at a frequency equal to the frequency of said firstswitching signal to provide a first gate output signal and a second gateoutput signal; applying said first gate output signal to a second audiogate and switching said second audio gate at a frequency equal to thefrequency of said second switching signal to provide said first andsecond audio signals; and applying said second gate output signal to athird audio gate and switching said third audio gate at a frequencyequal to the frequency of said second switching signal to provide saidthird and fourth audio signals.
 3. In a four channel broadcastingsystem, a method for providing a compatible four channel compositesignal for use in conjunction with an FM radio transmitter fortransmitting said composite signal, which composite signal includes amain channel with first, plus second, plus third and plus fourth audiosignals combined thereon, a pilot signal of a predetermined frequencyremoved from said main channel, a first suppressed sub-channel centeredon a frequency equal to the second harmonic of said pilot signal, saidfirst sub-channel having first, plus second, minus third, minus fourthaudio signals and in quadrautre therewith first, minus second, minusthird and plus fourth audio signals combined thereon and a secondsuppressed sub-channel centered on a frequency twice that of said firstsub-channel having first, minus second, plus third, minus fourth audiosignals combined thereon, comprising the steps of: providing a pilotsignal of a first predetermined frequency; providing a first switchingsignal at twice the frequency of said pilot signal; providing a secondswitching signal at quadruple the frequency of said pilot signal;utilizing said second switching signal to switch between said first andsecond audio signals to obtain a first signal; utilizing said secondswitching signal to switch between said third and fourth audio signalsto obtain a second signal; utilizing said first switching signal toswitch between said first and second signals to obtain a third signal;combining said third signal with said pilot signal to obtain saidcomposite signal; and modulating said FM transmitter with said compositesignal.
 4. The method according to claim 3 further comprising the stepsof: providing a main channel extending up to a frequency less than thefrequency of said pilot signal and having first, plus second, plusthird, plus fourth audio signals combined thereon; providing asuppressed sub-channel centered at twice the frequency of said pilotsignal and having first, plus second, minus third, minus fourth audiosignals and in quadrature therewith first, minus second, minus third andplus fourth audio signals combined thereon; and providing a secondsuppressed sub-channel at quadruple the frequency of said pilot signaland having first, minus second, plus third, minus fourth audio signalscombined thereon.
 5. In a four channel receiving system, a method forderiving first, second, third and fourth audio signals from a compatiblefour channel composite signal received from an FM radio transmitter,which composite signal includes a main channel with first, plus second,plus third and plus fourth audio signals combined thereon, a pilotsignal having a predetermined frequency removed from said main channel,a first suppressed sub-channel which is centered at the second harmonicof said pilot signal, said first sub-channel having first, plus second,minus third, minus fourth audio signals and in quadrature therewithfirst, minus second, minus third and plus fourth audio signals combinedthereon and a second suppressed sub-channel centered at the fourthharmonic of said pilot signal and having first, minus second, plusthird, minus fourth audio signals combined thereon comprising the stepsof: deriving a signal equal in frequency to the frequency of said pilotsignal; deriving a first switching signal equal in frequency to twicethe frequency of said pilot signal; deriving a second switching signalequal in frequency to quadruple the frequency of said pilot signal;utilizing said first switching signal to switch said composite sigNal attwice the frequency of said pilot signal to obtain a first signal and asecond signal; utilizing said second switching signal to switch saidfirst signal at quadruple the frequency of said pilot signal to obtainsaid first and second audio signals; and utilizing said second switchingsignal to switch said second signal at quadruple the frequency of saidpilot signal to obtain said third and fourth audio signals.
 6. In a fourchannel broadcasting system, a method for providing a four channelcomposite signal compatible with existing mono and stereo standards foruse in conjunction with an FM radio transmitter having a predeterminedcarrier frequency, comprising the steps of: providing a main channelextending up to a first predetermined frequency and having first, plussecond, plus third and plus fourth audio signals combined thereon;providing a pilot signal of a second predetermined frequency removedfrom said carrier frequency; providing a first sub-channel centered atthe second harmonic of the frequency of said pilot signal and containingtwo sub-carriers; modulating the first sub-carrier of said firstsub-channel with the plus first, plus second, minus third, minus fourthaudio signals and modulating in quadrature therewith the secondsub-carrier of said first sub-channel with plus first, minus second,minus third and plus fourth audio signals; providing a secondsub-channel centered at the fourth harmonic of the frequency of saidpilot signal and containing a sub-carrier; and modulating thesub-carrier of said second sub-channel with the plus first, minussecond, plus third and minus fourth audio signals.
 7. In a four channelbroadcasting system, a system for providing a compatible four channelcomposite signal for use in conjunction with an FM radio transmitterhaving said composite signal modulated on the carrier thereof andconsisting of a main channel, synchronizing pilot signal of a firstpredetermined frequency, a first sub-channel centered at the secondharmonic of said pilot signal and a second sub-channel centered at thefourth harmonic of said pilot signal comprising: means for providingfirst, second, third and fourth audio signals; means for providing apilot signal of a first predetermined frequency; means for providing afirst sub-carrier frequency signal at twice the frequency of said pilotsignal; means for providing a second sub-carrier frequency signal atquadruple the frequency of said pilot signal; a first audio gate; meansfor applying said first and second audio signals to said first audiogate; means for switching said first audio gate at the frequency of saidsecond sub-carrier frequency signal; a second audio gate; means forapplying said third and fourth audio signals to said second audio gate;means for switching said second audio gate at the frequency of saidsecond sub-carrier frequency signal; a third audio gate; means forapplying the outputs from said first and second audio gates to saidthird audio gate; means for switching said third audio gate at thefrequency of said first sub-carrier frequency signal; and means forcombining the output of said third audio gate with said pilot signal toobtain said composite signal.
 8. In a four channel receiving system, asystem for decoding first, second, third and fourth audio signals from acompatible four channel composite signal adapted to be received from anFM radio transistor having said composite signal modulated on thecarrier thereof, said composite signal consisting of a main channel, asynchronizing pilot signal of a first predetermined frequency, a firstsub-channel centered at the second harmonic of said pilot signal and asecond sub-channel centered at the fourth harmonic of said pilot signal,comprising: means for extracting said pilot signal from said compositesignal; means for providing a first switching signal at twice thefrequency of said pilot signal; means for providing a second switchingsignal at quadruple the frequency of said pilot signal; a first audiogaTe; means for applying said composite signal to said first audio gate;means for switching said first audio gate at a frequency equal to thefrequency of said first switching signal to provide a first gate outputsignal and a second gate output signal; a second audio gate; means forapplying said first gate output signal to said second audio gate; meansfor switching said second audio gate at a frequency equal to thefrequency of said second switching signal to provide said first andsecond audio signals; a third audio gate; means for applying said secondgate output signal to said third audio gate; and means for switchingsaid third audio gate at a frequency equal to the frequency of saidsecond switching signal to provide said third and fourth audio signals.9. In a four channel broadcasting system, a system for providing acompatible four channel composite signal for use in conjunction with anFM radio transmitter for transmitting said composite signal, whichcomposite signal includes a main channel with first, plus second, plusthird and plus fourth audio signals combined thereon, a pilot signal ofa predetermined frequency removed from said main channel, a firstsuppressed subchannel centerd on a frequency equal to the secondharmonic of said pilot signal, said first sub-channel having first, plussecond, minus third, minus fourth audio signals and in quadraturetherewith first, minus second, minus third and plus fourth audio signalscombined thereon and a second suppressed sub-channel centered on afrequency twice that of said first sub-channel having first, minussecond, plus third, minus fourth audio signals combined thereon,comprising: means for providing a pilot signal of a first predeterminedfrequency; means for providing a first switching signal at twice thefrequency of said pilot signal; means for providing a second switchingsignal at quadruple the frequency of said pilot signal; means forswitching between said first and second audio signals at quadruple thefrequency of said pilot signal to obtain a first signal; means forswitching between said third and fourth audio signals at quadruple thefrequency of said pilot signal to obtain a second signal; means forswitching between said first and third signals at twice the frequency ofsaid pilot signal to obtain a third signal; and means for combining saidthird signal with said pilot signal to obtain said composite signal. 10.The system according to claim 9 wherein said composite signal includes amain channel extending up to a frequency less than the frequency of saidpilot signal and having first, plus second, plus third, plus fourthaudio signal combined thereon; a suppressed sub-channel centered attwice the frequency of said pilot signal and having first, plus second,minus third, minus fourth audio signals and in quadrature therewithfirst, minus second, minus third and plus fourth audio signals combinedthereon; and a second suppressed sub-channel at quadruple the frequencyof said pilot signal and having first, minus second, plus third, minusfourth audio signals combined thereon.
 11. In a four channel receivingsystem, a system for deriving first, second, third and fourth audiosignals from a compatible four channel composite signal received from anFM radio transmitter, which composite signal includes a main channelwith first, plus second, plus third and plus fourth audio signalscombined thereon, a pilot signal having a predetermined frequencyremoved from said main channel, a first suppressed sub-channel which iscentered at the second harmonic of said pilot signal, said firstsub-channel having first, plus second, minus third, minus fourth audiosignals and in quadrature therewith first, minus second, minus third andplus fourth audio signals combined thereon and a second suppressedsub-channel centered at the fourth harmonic of said pilot signal andhaving first, minus second, plus third, minus fourth audio signalscombined thereon comprising: means for deriving a signal equal inFrequency to the frequency of said pilot signal; means for deriving afirst switching signal equal in frequency to twice the frequency of saidpilot signal; means for deriving a second switching signal equal infrequency to quadruple the frequency of said pilot signal; means forutilizing said first switching signal to switch said composite signal attwice the frequency of said pilot signal to obtain a first signal and asecond signal; means for utilizing said second switching signal toswitch said first signal at quadruple the frequency of said pilot signalto obtain said first and second audio signals; and means for utilizingsaid second switching signal to switch said second signal at quadruplethe frequency of said pilot signal to obtain said third and fourth audiosignals.
 12. In a four channel broadcasting system, a system forproviding a four channel composite signal compatible with existing monoand stereo standards for use in conjunction with an FM radio transmitterhaving a predetermined carrier frequency, comprising: means forproviding a main channel extending up to a first predetermined frequencyand having first, plus second, plus third and plus fourth audio signalscombined thereon; means for providing a pilot signal of a secondpredetermined frequency removed from said carrier frequency; means forproviding a first sub-channel centered at the second harmonic of thefrequency of said pilot signal and containing two sub-carriers; meansfor modulating the first sub-carrier of said first sub-channel with theplus first, plus second, minus third, minus fourth audio signals and formodulating in quadrature therewith the second sub-carrier of said firstsub-channel with plus first, minus second, minus third and plus fourthaudio signals; means for providing a second sub-channel centered at thefourth harmonic of the frequency of said pilot signal and containing asubcarrier; and means for modulating the sub-carrier of said secondsub-channel with the plus first, minus second, plus third and minusfourth audio signals.
 13. A compatible four channel audio system forproviding a compatible four channel composite signal comprising meansfor providing left front, left rear, right front and right rear audiosignals, means for providing a 19 kHz pilot signal, means for providinga 38 kHz sub-carrier signal, first circuit means coupled to said leftfront and left rear audio signals for combining all frequencies of saidleft front and left rear audio signals above 4 kHz and for providingseparation between said left front and left rear audio signals atfrequencies less than 4 kHz, first encoder means coupled to said firstcircuit means and said left front and left rear audio signals, saidfirst encoder means including first switch means responsive to saidpilot signal for providing a first encoder output during the positivehalf cycle of said pilot signal and second switch means responsive tosaid pilot signal for providing a second encoder output during thenegative half cycle of said pilot signal, second circuit means coupledto said right front and right rear audio signals for combining allfrequencies of said right front and right rear audio signals above 4 kHzand for providing separation between said right front and right rearaudio signals at frequencies less than 4 kHz, second encoder meanscoupled to said second circuit means and said right front and right rearaudio signals, said second encoder means including first switch meansresponsive to said pilot signal for providing a first encoder outputduring the positive half cycle of said pilot signal and said secondswitch means responsive to said pilot signal for providing a secondencoder output during the negative half cycle of said pilot signal,means for combining the output signals from said first and secondencoder means to provide said composite signal, and transmitter meansfor transmitting said composite signal, said transmitter meansresponsive to said 38 kHz sub-carrier signal for transmitting fortransmitting during the positive cycles of said subcarrier signal a mainchannel having left front plus right front audio signals modulatedthereon, said 19 kHz pilot signal and a suppressed sub-channel havingleft front minus right front audio signals modulated thereon and fortransmitting during the negative cycles of said sub-carrier signal amain channel having left rear plus right rear audio signals modulatedthereon, said 19 kHz pilot signal and a suppressed sub-channel havingleft rear minus right rear signals modulated thereon.
 14. The systemaccording to claim 13 further including means for receiving saidcomposite signal, means for extracting said 19 kHz pilot signal fromsaid composite signal, first decoder means adapted to receive the leftfront and left rear audio signals, and first decoder including firstswitch means responsive to said pilot signal for providing said leftfront audio signal during the positive half cycle of said pilot signaland second switch means responsive to said pilot signal for providingsaid left rear audio signal during the negative half cycle of said pilotsignal, and second decoder means adapted to receive said right front andright rear audio signals, said second decoder means including firstswitch means responsive to said pilot signal for providing said rightfront audio signal during the positive half cycle of said pilot signaland second switch means responsive to said pilot signal for providingsaid right rear audio signal during the negative half cycle of saidpilot signal.